gh-116307: Create a new import helper 'isolated modules' and use that…

… instead of 'Clean Import' to ensure that tests from importlib_resources don't leave modules in sys.modules.

importlib_resources/tests/compat/py312.py

@@ -0,0 +1,18 @@
+import contextlib
+
+from .py39 import import_helper
+
+
+@contextlib.contextmanager
+def isolated_modules():
+    """
+    Save modules on entry and cleanup on exit.
+    """
+    (saved,) = import_helper.modules_setup()
+    try:
+        yield
+    finally:
+        import_helper.modules_cleanup(saved)
+
+
+vars(import_helper).setdefault('isolated_modules', isolated_modules)

importlib_resources/tests/compat/py39.py

@@ -14,7 +14,6 @@ class import_helper:  # type: ignore
             modules_setup,
             modules_cleanup,
             DirsOnSysPath,
-            CleanImport,
         )
 
 

importlib_resources/tests/test_files.py

@@ -9,7 +9,8 @@
 from . import data01
 from . import util
 from . import _path
-from .compat.py39 import os_helper, import_helper
+from .compat.py39 import os_helper
+from .compat.py312 import import_helper
 
 
 @contextlib.contextmanager
@@ -68,7 +69,7 @@ def setUp(self):
         self.addCleanup(self.fixtures.close)
         self.site_dir = self.fixtures.enter_context(os_helper.temp_dir())
         self.fixtures.enter_context(import_helper.DirsOnSysPath(self.site_dir))
-        self.fixtures.enter_context(import_helper.CleanImport())
+        self.fixtures.enter_context(import_helper.isolated_modules())
 
 
 class ModulesFilesTests(SiteDir, unittest.TestCase):